Ultrasonic transducers are often used as impulse mode transducers operating over a wide range of frequencies. Since such transducers need to handle wideband frequency signals, wideband design is an important subject. In the prior art, impedance converters (also known as impedance matching layers) have been placed on a face of a piezoelectric element or piezoelectric active layer (also called a “piezoelectric array” herein) of an ultrasonic transducer to improve the wideband frequency response of the transducer. One of the important applications of wideband transducers is in medical imaging systems. Economical, reliable and reproducible mass-production processes for transducers for use in medical imaging systems are particularly desirable.
Impedance converters for ultrasonic transducers are known in the art. As is known in the art, an ultrasonic transducer includes a piezoelectric active layer, one or more front matching layers on a front face of the piezoelectric active layer to serve as an impedance converter, and a backing absorber on a rear face of the piezoelectric active layer. A typical piezoelectric material, such as lead zirconate titanate has high characteristic acoustic impedance, for example, Zpiezoelectric array=30×106 kg/m2s (Rayl). A typical propagation medium, such as water, has low characteristic acoustic impedance, for example, ZR=1.5×106 Rayl. Because of the difference in characteristic acoustic impedances of these media, acoustic waves in the piezoelectric active layer of an ultrasonic transducer are reflected backward into the piezoelectric active layer at the boundary between the piezoelectric active layer and the transmission medium (the front boundary) and reflected frontward into the piezoelectric active layer at the back boundary (the boundary between the rear face of the piezoelectric active layer and the material to the rear of the piezoelectric active layer). This results in a resonance at a specific frequency in the ultrasonic transducer, as determined by the half wavelength condition of the piezoelectric material.
When such a resonated transducer is driven by a voltage pulse (when acting as a transmitter) or by an acoustic pulse (when acting as a receiver), the signal wave does not decay quickly (a phenomenon known as ringing). This effectively renders such a transducer unsuitable for imaging systems, in which systems short acoustic pulse beams are excited, directionally scanned and reflected back from a target to enable an image of the target to be constructed. A front impedance conversion layer (also known in the art as a matching layer for reducing reflections) is inserted between the front face of the piezoelectric layer and the propagation medium to mitigate creation of resonance due to the difference in the characteristic acoustic impedances of the piezoelectric material and the front propagation medium.
A piezoelectric layer's vibration excites an acoustic wave in the backward direction, i.e., in a direction away from the front face of the piezoelectric layer. A certain amount of reflection from the back boundary towards the front face may be desirable to improve the sensitivity of the ultrasonic transducer. Often a backing absorber layer of acoustic absorber material is attached to the rear face of the piezoelectric layer. If the characteristic acoustic impedance of the backing absorber material effectively matches that of the piezoelectric material, a significant amount of acoustic wave energy passes through the back boundary without reflection and is absorbed by the backing absorber layer. In such a case, the sensitivity of the transducer is lowered and the bandwidth may become excessive for some applications. Therefore, some mismatch between the characteristic acoustic impedance of the piezoelectric material and the backing absorber material is desirable, depending on the required bandwidth and sensitivity.
The characteristic acoustic impedance of the backing absorber material may be selected to obtain a desired performance of the ultrasonic transducer. If a transducer cannot be provided with a backing absorber material of a suitable characteristic acoustic impedance, a back impedance conversion layer may be added between the piezoelectric active layer and the backing absorber layer to provide a desired overall acoustic impedance at the back boundary of the piezoelectric layer.
A typical acoustic impedance conversion structure may be a layer of uniform thickness, the thickness equal to about one-quarter of the wavelength of a desired operating wavelength of the acoustic transducer. Another known acoustic impedance conversion structure providing still wider bandwidth uses double matching layers. It is quite difficult to obtain appropriate materials for these layers while satisfying the specific designed values of the characteristic acoustic impedances. A suitable structure is described in U.S. Patent Publication No. 2011/0050039 to Toda, et al., which is fully incorporated by reference herein.
A problem associated with the conventional design of ultrasonic transducers arises in the design of the structure for the transducer return signal. The prior art structure for routing the transducer return signal typically involves painstaking labor to connect the piezoelectric/polymer array to the return lines. Furthermore, because piezoelectric materials are temperature sensitive, conventional methods to make electrical connections like solder cannot be used to create the return signal paths. Thus, the prior art method of creating a return signal path is both difficult and labor intensive.